Normally, a baby inherits 23 chromosomes from each parent, for a total of 46. But when the egg or sperm cell is developing, there could be errors, which can result in a fetus having 47 chromosomes instead of 46. That means instead of having 23 pairs of chromosomes, a baby has 22 pairs plus a set of three, which is known as trisomy (three copies of one chromosome).
In most cases, babies with trisomy are miscarried very early in the first trimester. According to the March of Dimes, more than 50 percent of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo.
The most common chromosomal abnormality is Down syndrome (trisomy 21), which affects 1 in 800 babies. If your baby has Down syndrome, it means he has three copies of chromosome 21. While all babies with Down syndrome have some degree of mental retardation, in many cases, it’s not too severe. Other common trisomies are trisomy 13 and 18 — these are almost always associated with severe mental retardation and congenital anomalies. Most babies with these abnormalities sadly die before they turn one.
While you can’t do anything before or during pregnancy to prevent baby from developing an extra chromosome, the risk of it happening increases with the mom’s age. Prenatal tests like an amniocentesis or a chorionic villus sampling (CVS) can diagnose chromosomal abnormalities during pregnancy. After baby is born, a blood test can be performed to check for abnormalities. The American College of Obstetricians and Gynecologists recommends that all pregnant women get screening tests to indicate whether their baby has a high risk of developing a chromosomal abnormality. Moms-to-be can get a blood test done in the first trimester (combined with a special ultrasound to analyze the back of baby’s neck) or second trimester. If a pregnant woman has an abnormal screening test result, her doctor might recommend that she get an amniocentesis or CVS.
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